What's going on with my .338WM trajectory?

I loaded up some 250gn Hornady BTHP match projectiles and chronographed them at 2652 fps.Checked my zero at 100 yards was spot on, then worked out a drop at 1000 yards of 30.75 MOA and at 605 yards of 13.75 MOA.Temp 19C pressure 1000mb. Rifle is a Tikka T3 Laminate with a Clearidge 2.5-12.5x42 Ultra XP5 on 20MOA rail. It is quite an accurate rifle for a light weight hunting rifle.

Now at 1000 yards the group was 44cm high and at 605 yards the group was 30cm high. I have played about with the inputs into my ballistic program but can not get thing to line up like they should. I am thinking as the height error seems to be linear from 605 to 1000, that maybe my scope turret adjustments are not quite MOA?????

Yes I did input the barometric pressure and temp into the ballistics targeting software program.

It was overcast when I did my velocity testing and I have a sky screen over the chronograph to diffuse the light.

I have played about with the inputs into the program. I tried inputting, higher velocities and changes to my atmospheric conditions but can not get both the 605 and 1000 yard drops to agree with the actual measured drops from shooting.

The area I shoot is well measured out and I have no problem with my drop figures agreeing with what my .338 Edge shoots over the same distances.

Well I went out and tested the scope on a large piece of paper at 100 yards and found it to be out by about 8% in its elevation adjustment.
i.e. if I dial 20MOA I actually get 21.5MOA of movement.
This is quite repeatable and is not due to backlash as none was detected.

So it looks like I will have to come up with a drop chart for this particular scope as my balistic program only has IPHY and MOA not MOA+8%......LoL.

Well I went out and tested the scope on a large piece of paper at 100 yards and found it to be out by about 8% in its elevation adjustment.
i.e. if I dial 20MOA I actually get 21.5MOA of movement.
This is quite repeatable and is not due to backlash as none was detected.

So it looks like I will have to come up with a drop chart for this particular scope as my balistic program only has IPHY and MOA not MOA+8%......LoL.

It is a nice scope to look through, so it is a pity that they could not get the elevation turret a bit more accurate.I read somewhere that the industry standard for turret accuracy of movement, is 2.5% so it is a bit disapointing that this scope is just a bit less than 10%.

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